Who is Bill Nelson?

The release of Florida Democratic Sen. Bill Nelson’s first ad Wednesday, titled “40 years,” receives this headline greeting from The Miami Herald: “Why is Sen. Bill Nelson reminding us of who he is?”

It’s a good question. Nelson has been a player in Florida politics for more than four decades, with service in the state House, the U.S. House and two terms in the Senate. He’s a fifth-generation Floridian, has run for statewide office countless times and has a rare claim to fame: he’s one of only two sitting congressmen in American history to have been in outer space.

The ad, well produced and hitting all the centrist notes he likes to stress, is almost on par with an introductory ad from a political unknown, which is odd considering he has held office on and off for about, well, 40 years.

But there's a reason Nelson needs to remind people of who he is and what he has done: He has been shredded by about $7 million worth of negative ads from conservative third-party groups that want Republican Rep. Connie Mack to win and help put the Senate back in GOP hands. Nelson clings to a slim lead in the polls. The race is a toss-up right now. If it wasn't, Nelson would be saving his money.

The carpet-bombing ad campaign is only part of Nelson’s problem, though. For all his history in Florida politics, he’s still surprisingly unknown: According to a poll released Wednesday by the Democratic firm Public Policy Polling, 26 percent of Florida voters said they weren’t sure about his job performance.

That oddly low level of familiarity has been fairly consistent in the polls — when Quinnipiac University asked voters in January whether they had a favorable or unfavorable view of the two-term senator, 35 percent said they hadn’t heard enough about him to say.

Republicans make the argument that it says something about Nelson’s record, but perhaps more important, it reveals something about the difficult nature of running for statewide office in the nation’s third-largest state — a place with 10 major media markets and a churning and highly transient population.